The present invention relates to a turbine nozzle, and more particularly to a turbine nozzle having an array of nozzle blades disposed circumferentially in an annular passage defined between an inner ring and an outer ring of a diaphragm and fixed to the inner and outer rings of the diaphragm.
It has been recognized in recent years that it is important to improve the performance of a turbine in order to improve energy consumption for mechanical operation or improve the efficiency of power generation in a power generating plant.
In order to improve the performance of a turbine, it is necessary to reduce the internal losses in each of the turbine stages. The internal losses in each of the turbine stages include a blade profile loss, a secondary flow loss, and a leakage loss.
The proportion of the secondary flow loss is large in a turbine stage where an aspect ratio (blade height/blade chord) is small and a blade height is small. Therefore, it is effective to reduce the secondary flow loss for thereby improving the performance of the turbine.
The mechanism of generation of the secondary flow will be described below.
As shown in FIG. 15 of the accompanying drawings, a flow G flowing in between nozzle blades 1 is subject to a force caused by a pressure gradient from a pressure surface F to a suction surface B in each of the nozzle blades 1. In a main flow-away from a turbine end wall, the force caused by the pressure gradient and a centrifugal force caused by the turning of the flow are in balance. However, a flow in a boundary layer near the turbine end wall has a low level of kinetic energy, and hence is carried from the pressure surface F to the suction surface B by the force caused by the pressure gradient as indicated by the arrows J. In a latter half of the flow passage, the flow collides with the suction surface B and rolls up, thus forming a flow passage vortex W. The flow passage vortex W accumulates a low-energy fluid in the end wall boundary layer to thereby generate a non-uniform energy distribution downstream of the nozzle blade. Although the non-uniform energy distribution is uniformized downstream of the nozzle blade, a large energy loss is generated during its uniformization. In FIG. 15, E represents a radial line, and L represents a hub end wall.
Various attempts have heretofore been made to suppress the above secondary flow.
For example, as shown in FIG, 16 of the accompanying drawings, bales 1 are inclined at an angle xcex8 to the radial line E for thereby weakening any blade-to-blade pressure gradient near the hub end wall of the blade. In FIG. 16, reference numeral 2 represents an outer ring, and reference numeral 3 represents an inner ring. Further, as shown in FIGS. 17 and 18 of the accompanying drawings, nozzle blades 1 are curved at their opposite ends to orient the pressure surfaces F to the end wall. In FIG. 17, U represents an outer diameter surface. In FIG. 18, xcex8t represents the angle between the tangent to the blade stacking line 1 at the tip end wall and radial line E, xcex8r represents the angle between the tangent to the blade stacking line 1 at the hub end wall and radial line E, and h represents a blade height. According to the conventional attempts, while the same blade profile is employed, blade stacking lines are cured or inclined in a direction to weaken the blade-to-blade pressure gradient near the end walls, thereby controlling the secondary flow to reduce the loss.
Another conventional technology involves an inclined or curved surface imparted to a nozzle blade across its entire height for thereby controlling the secondary flow, as disclosed in Japanese laid-open patent publication No. 10-77801.
In order to control the pressure gradient with the above conventional arrangements, the nozzle blade needs to be largely inclined or curved, and hence efforts to meet such a requirement tend to cause problems in the manufacturing process or in the mechanical strength of the nozzle blades.
Further, according to such curved or inclined blades, a flow distribution at the outlet of the blades is liable to differ greatly from a flow distribution on blades which are neither curved nor inclined.
For example, FIG. 19 of the accompanying drawings shows a graph having a horizontal axis representative of positions along the height of a blade, which are expressed as a dimensionless ratio with respect to the height h, and a vertical axis representative of circumferential velocities Vt and meridional velocities Vm, which are expressed as a dimensionless ratio with respect to the absolute velocity V (=(Vt2+Vm2)0.5). The graph shown in FIG. 19 indicates that flow velocity distributions of an ordinary blade (indicated by the solid-line curves) and those of a curved blade (indicated by the broken-line curves) differ at the opposite ends of the blades.
If nozzle blades are of a curved shape and are combined with conventional rotor blades positioned downstream of the nozzle blades, then flows from the nozzle blades do not match the rotor blades, and the curved nozzle blades may not be effective. In such a case, new rotor blades capable of matching flows from the outlet of the curved nozzle blades are required, and thus such an arrangement cannot meet a wide range of applications.
It is therefore an object of the present invention to provide a turbine nozzle which is capable of reducing a secondary flow loss and producing an outlet flow that is the same as an outlet flow from ordinary blades, and does not adversely affect rotor blades positioned downstream of the turbine nozzle.
According to one aspect of the present invention, there is provided a turbine nozzle comprising: an array of nozzle blades (1) disposed circumferentially in an annular passage (4) defined between inner and outer rings of a diaphragm and fixed to the inner and outer rings of the diaphragm; and a flow passage defined between a pressure surface (F) and a suction surface (B) of adjacent ones of the nozzle blades, a cross section of the flow passage including predetermined ranges extending along a blade height from the inner and outer diameter surfaces (hub and tip end walls) and defined by a curved line, and another range defined by a substantially straight line.
Since the cross section of the flow passage in the predetermined ranges on the pressure surface and the suction surface includes a region defined by the curved line and a region defined by the substantially straight line, the turbine nozzle according to the present invention is clearly different in structure from the nozzle blade disclosed in Japanese laid-open patent publication No. 10-77801.
According to another aspect of the present invention, there is also provided a turbine nozzle comprising: an array of nozzle blades (1) disposed circumferentially in an annular passage (4) defined between inner and outer rings of a diaphragm and fixed to the inner and outer rings of the diaphragm; a pressure surface (F) in each of the nozzle blades facing the tip end wall of the turbine diaphragm in a predetermined range in the meridional direction of the nozzle blade and in a predetermined range between the tip end wall and a midspan of a blade, and the pressure surface facing the hub end wall of the turbine diaphragm in a predetermined range between the hub end wall and the midspan of the blade; a suction surface (B) in each of the nozzle blades facing the hub end wall of the turbine diaphragm in a predetermined range in the meridional direction of the nozzle blade and in a predetermined range between the tip end wall and a midspan of the blade, and the suction surface facing the tip end wall of the diaphragm in a predetermined range between the hub end wall and the midspan of said blade.
Here, the predetermined range may comprise a range corresponding to at least 30% of a meridional width (Cx) of the nozzle blade from a leading edge (1f) of the nozzle blade in a meridional direction (x). The predetermined range may comprise a range corresponding to 20 to 40% of the blade height (h) from the hub end wall (L) of the nozzle blade (1), and a range corresponding to 20 to 40% of the blade height (h) from the tip end wall (U) of the nozzle blade (1).
In the above predetermined ranges, the pressure surface (F) of the nozzle blade (1) is arranged to face the tip end wall at the tip end wall side, i.e., is curved to face the tip end wall, and is arranged to face the hub end wall at the hub end wall side, i.e., is curved to face the hub end wall, and the suction surface (B) of the nozzle blade (1) is arranged to face the hub end wall at the tip end wall side, i.e., is curved to face the hub end wall, and is arranged to face the tip end wall at the hub end wall side, i.e., is curved to face the tip end wall.
A line (1p) on the pressure surface and a line (1s) on the suction surface along the height of the nozzle blade (1) have central portions (S) which are preferably defined by substantially straight lines except for the range (C1) corresponding to 20 to 40% from the hub end wall (L) along the height (h) of the nozzle blade (1) and the range (C2) corresponding to 20 to 40% from the tip end wall (U) along the height (h) of the nozzle blade (1). Specifically, a line on the pressure surface (F) and a line on the suction surface (B) in the cross section of the flow passage in an arbitrary meridional position in a range of at least 30% from a leading edge (1f) of the nozzle blade along a meridional width (Cx) of the nozzle blade have central portions which are preferably defined by substantially straight lines except for the range (C1) corresponding to 20 to 40% from the hub end wall (L) along the height (h) of the nozzle blade (1) and the range (C2) corresponding to 20 to 40% from the tip end wall (U) along the height (h) of the nozzle blade (1).
The cross section of the flow passage is defined by a line on said pressure surface (F) and a line on said suction surface (B) in a meridional position within a range of at least 30% from a leading edge (1f) of the nozzle blade (1) along a meridional width (Cx) of the nozzle blade (1), each of the lines comprising a substantially straight line in a central region of the nozzle blade.
The distance (Sh) from an intersection (Pt1) between the line (C1) on the pressure surface or the suction surface and the hub end wall (L) to an intersection (Pc1) between an extension (SE1) of the central portion (S) on the pressure surface or the suction surface defined by the substantially straight line and the hub end wall (L), and the distance (St) from an intersection (Pt2) between the line (C2) on the pressure surface or the suction surface and the tip end wall (U) to an intersection (Pc2) between an extension (SE2) of the central portion (S) and the tip end wall (U) have a maximum value at the leading edge (1f) of the nozzle blade, and at least 4% of the blade height (h) in a position at 30% of the meridional width from the leading edge of the nozzle blade.
The maximum value of the distances (Sh, St) at the leading edge (1f) of the nozzle blade (1) should be preferably in the range of from 5 to 15% of the blade height (h).
If the distance between the intersections from the leading edge (1f) of the nozzle blade to a position at 55-65% of the meridional width is represented by Sh or St, the nozzle height is represented by h, and the ratio of the meridional distance from the leading edge (1f) of the nozzle blade to the blade width (Cx) is represented by xcex9, then the following equation should preferably be satisfied:
St/h, Sh/h=xcexa3Anxc2x7xcex9n
where An represents a coefficient and n is an integer of 0 or greater.
In the above equation, a higher-order term which is substantially zero is negligible. In other words, n is an integer of 0 or greater which is of a numerical value including all higher-order terms that are not negligibly small.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.